Problem: $ C = \left[\begin{array}{rrr}1 & 0 & 0 \\ 1 & 1 & 2 \\ 2 & 1 & 1\end{array}\right]$ What is $ C^{-1}$ ?
Solution: $ = \left[\begin{array}{rrr}1 & 0 & 0 \\ -3 & -1 & 2 \\ 1 & 1 & -1\end{array}\right]$